Reorg alarms controller to facilitate history API

Use controller/sub-controller pattern for the alarms collection and
individual alarms respectively (as we do for meters).

This will facilitate the addition of a 'history' custom action on
individual alarms.

Precursor to support for bp alarm-audit-api

Change-Id: I8e43cebdc82f05b2cc427c7a27114101836dbf5f